Conventionally, there has been an apparatus for detecting a decrease in tire air pressure for detecting, based on rotation (wheel speed) information regarding four tires attached to a vehicle, a decreased pressure of the tires. This apparatus uses a principle according to which a tire having a decreased pressure shows a reduced outer diameter (tire dynamic loaded radius) compared to that of tires having a normal air pressure and thus shows an increased rotation speed and an increased rotation angular velocity compared to those of other normal tires. In the case of a method of detecting a reduced internal pressure based on a relative difference in the rotation angular velocity of tires (see Patent Literature 1 for example) for example, a determination value DEL′ obtained byDEL′={(F1+F4)/2−(F2+F3)/2}/{(F1+F2+F3+F4)/4}×100(%)is used. In the formula, F1 to F4 represent rotation angular velocities of a left front wheel tire, a right front wheel tire, a left rear wheel tire, and a right rear wheel tire, respectively.
However, since the above method determines a decreased pressure by the determination value DEL′ that is based on the difference in sums of the rotation angular velocities of diagonal wheels among four wheels, the method cannot determine simultaneous deflation of two front tires or simultaneous deflation of two rear tires. This causes a disadvantage where a driver continuously drives the vehicle while not knowing that the tires have a decreased pressure and thus the fuel consumption is deteriorated due to an increased rolling resistance of the tires and the tires may burst.
Thus, there have been suggested various methods in order to judge simultaneous deflation of two front tires or two rear tires described above. In the case of Patent Literature 2 for example, the rotation information of two front tires of a vehicle is compared with the rotation information of two rear tires to determine, based on the relation between the resultant comparison values and a predetermined threshold value, whether a simultaneous deflation of two front tires or two rear tires occurs or not.